1. Technical Field of the Invention
The present invention relates generally to diagnosis methods for solenoid valves and common rail fuel injection systems for diesel engines. More particularly, the invention relates to a method of diagnosing a solenoid valve employed in a common rail fuel injection system for a diesel engine.
2. Description of the Related Art
There is known a diagnostic apparatus for diagnosing a plurality of solenoid-type Suction Control Valves (SCVs) that are employed in a fuel supply unit of a common rail fuel injection system for a diesel engine to control an amount of pressurized fuel supplied from the fuel supply unit to a common rail of the system.
The diagnostic apparatus is configured to electrically diagnose the SCVs. More specifically, the diagnostic apparatus detects the presence of drive current in each of the SCVs and diagnoses whether the SCV is in normal or abnormal condition based on the detection results.
Accordingly, the diagnostic apparatus is only capable of detecting electrical failures or abnormalities, such as disconnection within each of the SCVs, disconnection within an EDU (Electronic Drive Unit) for driving the SCVs, and disconnection between each of the SCVs and the EDU.
However, even without electrical failures, the SCVs still cannot normally operate when there exist mechanical failures or abnormalities in the SCVs. For example, when foreign matters flow into the SCVs along with fuel, they may cause the SCVs to be locked, thus making it impossible to suitably control the amount of pressurized fuel supplied from the fuel supply unit to the common rail.
By way of example, suppose that the fuel supply unit includes two alternately-activated SCVs, each of which is mounted to a high-pressure pump of the fuel supply unit to control the amount of fuel sucked into the pump.
Then, when one of the SCVs is mechanically locked, the locked condition may not be easily perceived in low-flow conditions because there is the other SCV normally operating. On the other hand, in high-flow conditions, such as full acceleration, high-speed running, and hill climbing, the locked condition will cause shortage of the pressurized fuel. Thus, it is possible to detect the locked condition of the SCV by running the engine under a high-flow condition.
However, in a service shop, it is generally difficult to run the engine under a high-flow condition and it is thus difficult to diagnose the SCVs.
Further, even if it is possible to run the engine under a high-flow condition, it is still difficult for a diagnostician to accurately identify the locked SCV relying only on feeling and a diagnosis of fuel leakage.
Furthermore, in some cases, running the engine under a high-flow condition may temporary recover the locked SCV to normal and thus no any additional measure is taken. However, in other flow conditions, the SCV may be easily locked again, thus significantly impairing the drivability of a vehicle that employs the common rail fuel injection system.